VISUALIZING BRAIN ASTROCYTES - CHARACTERIZATION OF 3H-BU99008, AN IMIDAZOLINE 2 RECEPTOR LIGAND, IN THE HUMAN AD BRAIN

Abstract

Since glia activation and neuroinflammation are recognized as early events in Alzheimer's disease (AD), glia seems to be a promising therapeutic target in AD. Specifically, astrocytosis has been shown to be an early phenomenon in AD, even at a presymptomatic stage (Rodriguez-Vieitez et al., Brain 2016;139(Pt 3):922-36). However, few PET tracers are available for investigating in vivo astrocyte activation, therefore there is a growing interest in developing new astrocytic tracers. Presently, the only PET tracer used for the imaging of astrocyte reactivity in AD is [11C]-deuterium-L-deprenyl ([11C]-DED). [11C]-DED binds to the enzyme monoamine oxidase-B (MAO-B), which has shown to be overexpressed in reactive astrocytes. The aim of our study is to characterize a tentative astrocytic PET tracer, BU99008, in the assessment of reactive astrocytes and neuroinflammation in AD, and to compare its characteristics with the astrocytic PET tracer [11C]-DED. BU99008 has shown to bind astrocytic mitochondrial imidazoline2 receptors (I2R), which have been linked to several central nervous system disorders, including AD. Most interestingly, the density of I2R was shown to be elevated in postmortem AD brain with approximately 63%. For the in vitro characterization of [3H]-BU99008, both competition and saturation binding assays are performed in postmortem brain tissue from both AD patients and cognitive normal control (CN). In addition, autoradiography is performed to visualize regional distribution of [3H]-BU99008. The results are compared with results for [3H]-DED binding and competition studies. Preliminary results from [3H]-BU99008 in vitro competition binding studies shows specific binding in brain homogenates for both AD and CN, however with some differences in the nanomolar range. [3H]-DED showed specific binding in the nanomolar range in the same AD brain homogenates. To be able to detect astrocytosis early in AD, specific astrocyte biomarkers are needed. Our preliminary data shows that BU99008, a tentative astrocytic PET tracer, shows binding in CN and AD brain tissue in the nanomolar range, however more characterization is needed to understand its binding properties and future use in comparison to existing astrocytic biomarkers.